Consider the beginnings of the nascent Church.
Christ is born and the shepherds have begun their vigil. They round up the flocks of the nations, nations that formerly lived like cattle. They gather them into the Lord’s enclosure, lest these flocks suffer the assaults of spiritual wild beasts lurking concealed in the night’s darkness. And the shepherds keep good watch, having received their formation from the Good Shepherd.
The flock, then, is the people; the night is the world; the shepherds, bishops. Perhaps, too, he could be a shepherd to whom it is said in the Apocalypse “Be watchful and strengthen…” For the Lord has not only disposed that bishops watch over the flock, but has also given His Angels charge over it.